The present invention relates to the multicast relay technique in the Internet and in particular, to the multicast path building technique and the load splitting technique in the multicast network using the equal-cost multipath.
The multicast is a technique used in the Internet by a router to copy and output a packet to a plurality of lines so that one transmitted packet is simultaneously transmitted to a plurality of receivers. Since it is possible to transmit a packet with a small quantity of packet to a plurality of transmission destinations as compared to the transmission of a packet to one transmission destination by a plurality of times, the multicast technique is appropriately used in a real time medium requiring a large-capacity traffic such as streaming and teleconference.
In order to transmit a multicast packet only to the users who are to receive the packet, the router should select an appropriate line for outputting the multicast packet to a plurality of lines. The line selection judgment standard is created by a multicast route control protocol such as the PIM-SM (Protocol Independent Multicast Sparse-Mode) (see IFTF, “Protocol Independent Multicast Sparse-Mode (PIM-SM): Protocol Specification”, RFC2382).
In the PIM-SM path control calculation, the RPF (Reverse Path Forwarding) calculation plays a significant role. In the RPF, when the router receives from a downstream reception terminal, a request for participating in a multicast group from a certain multicast transmission server, the router calculates an adjacent PIM-SM router (RPF Neighbor) for reaching the multicast transmission server according to a unicast path table (hereinafter, this calculation is referred to as “RPF Neighbor calculation”) and sends the received request for participating in the group as a PIM Join message to the RPF Neighbor. Moreover, the router registers the path of the inverse direction with respect to the direction in which the request for participating in the multicast as a multicast packet relay path. With this repetition, all the requests for participating in the multicast are reported to all the routers between the transmission server and the reception terminal and the path of the inverse direction is registered as the multicast packet relay path. Accordingly, the multicast packet from the transmission server flows in the direction where the request is made for participating in the multicast packet.
On the other hand, in the unicast, ECMP (Equal Cost Multipath) technique is used for splitting a load on a router and a line. According to this technique, a plurality of adjacent routers are specified for one destination, and one adjacent router is elected as a relay destination in accordance with the content. By selecting a relay device for each stream at random, it is possible to split the load on the router and the line in stream unit. For the random selection for each stream, the Hash value of the stream information (such as a transmission source, a transmission destination address, and a port number) is normally used.
When operating the multicast by a network using the ECMP, there is a case that a plurality of RPF Neighbors are found as a result of the RPF Neighbor calculation. In this case, the PIM-SM specification makes the adjacent router having the maximum IP address to be the RPF Neighbor. Accordingly, there is a problem that even in the network using the ECMP, the multicast traffic is always concentrated in a particular router and the load splitting cannot be performed.
In order to solve this problem, the Cisco Co., Ltd. (see Cisco Systems “Load Splitting IP Multicast Traffic over ECMPO”, http://www.cisco.com/univercd/cc/td/doc/product/software/ios124/124tcg/timc_c/mcbspltt.htm#wp1053023) has developed a technique for selecting an RPF Neighbor by using a multicast transmission server IP address, a multicast group address and a Hash of an adjacent router address. By using this technique, it is possible to split the multicast traffic to a certain degree for each stream by using Hash value even in the network using the ECMP.